Allow ECK-Operator to easily watch deployed namespace

[developer67890]

This change adds a Helm Chart value singleNamespaceMode which configures to the operator to watch its deployed namespace if true.

While this could have been achieved by including the deployed namespace in the managedNamespaces list, it requires a user to specify a second time this namespace.

helm install elastic-operator ./eck-operator -n my-namespace --create-namespace \
  --set=managedNamespaces='{my-namespace}' \
...

This model is fragile and prone to mistyping or omissions. Other products have accomplished a similar feature by including something like clusterWide=false, which will result in the same behavior this PR achieves.

Changes:

• add singleNamespaceMode value to values.yaml, including short comment explanation
• if singleNamespaceMode = true, only include .Release.Namespace in the namespaces field of the Configmap.
• update documentation for "Install the operator restricted to a single namespace"

Resolves #8828

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[barkbay]

Should we validate that managedNamespaces is not set if singleNamespaceMode is true?
Something along the lines of:

Suggested change

	{{- if .Values.singleNamespaceMode }}
	{{- if and .Values.singleNamespaceMode .Values.managedNamespaces }}
	{{- fail "managedNamespaces should not be set when singleNamespaceMode is enabled" }}
	{{- end }}
	{{- if .Values.singleNamespaceMode }}

[barkbay]

Let's also create some unit tests in deploy/eck-operator/templates/tests/configmap_test.yaml:

# yaml-language-server: $schema=https://raw.githubusercontent.com/helm-unittest/helm-unittest/main/schema/helm-testsuite.json
suite: test configmap
templates:
  - configmap.yaml
tests:
  - it: should render configmap with default values
    asserts:
      - isKind:
          of: ConfigMap
      - equal:
          path: metadata.name
          value: elastic-operator

  - it: should set namespaces to release namespace when singleNamespaceMode is enabled
    set:
      singleNamespaceMode: true
    release:
      namespace: my-namespace
    asserts:
      - isKind:
          of: ConfigMap
      - matchRegex:
          path: data.eck\.yaml
          pattern: "namespaces: \\[my-namespace\\]"

  - it: should set namespaces from managedNamespaces when singleNamespaceMode is disabled
    set:
      singleNamespaceMode: false
      managedNamespaces:
        - ns1
        - ns2
        - ns3
    asserts:
      - isKind:
          of: ConfigMap
      - matchRegex:
          path: data.eck\.yaml
          pattern: "namespaces: \\[ns1,ns2,ns3\\]"

  - it: should fail when both singleNamespaceMode and managedNamespaces are set
    set:
      singleNamespaceMode: true
      managedNamespaces:
        - ns1
    asserts:
      - failedTemplate:
          errorMessage: "managedNamespaces should not be set when singleNamespaceMode is enabled"

  - it: should not set namespaces when neither singleNamespaceMode nor managedNamespaces are set
    set:
      singleNamespaceMode: false
      managedNamespaces: []
    asserts:
      - isKind:
          of: ConfigMap
      - notMatchRegex:
          path: data.eck\.yaml
          pattern: "namespaces:"

  - it: should set metrics port correctly
    set:
      config:
        metrics:
          port: 9090
          secureMode:
            enabled: false
    asserts:
      - isKind:
          of: ConfigMap
      - matchRegex:
          path: data.eck\.yaml
          pattern: "metrics-port: 9090"

  - it: should fail when metrics secureMode is enabled but port is 0
    set:
      config:
        metrics:
          port: 0
          secureMode:
            enabled: true
    asserts:
      - failedTemplate:
          errorMessage: "config.metrics.port must be greater than 0 when config.metrics.secureMode.enabled is true"

  - it: should set container registry correctly
    set:
      config:
        containerRegistry: my-registry.example.com
    asserts:
      - isKind:
          of: ConfigMap
      - matchRegex:
          path: data.eck\.yaml
          pattern: "container-registry: my-registry.example.com"

  - it: should set setDefaultSecurityContext correctly
    set:
      config:
        setDefaultSecurityContext: "true"
    asserts:
      - isKind:
          of: ConfigMap
      - matchRegex:
          path: data.eck\.yaml
          pattern: "set-default-security-context: true"

[barkbay]

@developer67890 sorry for the very late review, I think I'm 👍 with your proposal. I let a comment about adding a validation and some unit tests. Let me know what you think.